Carbon-bearing Molecules in a Possible Hycean Atmosphere

Brief

Madhusudhan et al. (2023) observed two transits of K2-18b (8.63 ± 1.35 M⊕, 2.61 ± 0.09 R⊕) using JWST NIRSpec G395H (5.3 hr, Jan 2023) and NIRISS SOSS (4.9 hr, Jun 2023) across 0.9–5.2 μm, yielding 4,411 total spectral data points. Bayesian atmospheric retrieval confirms CH4 at 5σ and CO2 at 3σ, both at ~1% volume mixing ratio in an H2-rich atmosphere, while NH3 is not detected, a chemical pattern consistent with theoretical predictions for a liquid-water ocean underlying a shallow H2-rich envelope. A tentative spectral feature consistent with dimethyl sulfide (DMS) is noted but not claimed as a firm detection, and multiple non-Hycean interior structures remain consistent with the planet's bulk properties.

Metadata

Category

Search

Venue

The Astrophysical Journal Letters

Type

Peer-reviewed

Year

2023

Authors

Nikku Madhusudhan, Subhajit Sarkar, Savvas Constantinou, Mans Holmberg, Anjali A. A. Piette, Julianne I. Moses

DOI

10.3847/2041-8213/acf577

arXiv

2309.05566

Access

Open access

Length

5.3 M

Programs

JWST GO Program 2722

Instruments

JWST NIRSpec G395H, JWST NIRISS SOSS

Data sources

JWST transmission spectroscopy of K2-18b

Tags

biosignature, astrobiology, exoplanet-atmosphere, Hycean-world, transmission-spectroscopy, JWST, sub-Neptune, DMS

Key points

• CH4 detected at 5σ and CO2 at 3σ confidence, both at ~1% volume mixing ratio in a H2-rich atmosphere, the first robust multi-molecule carbon detection in a sub-Neptune transmission spectrum.p.1
• NH3 non-detection is chemically consistent with predictions for a liquid-water ocean suppressing ammonia beneath a temperate H2-rich atmosphere.p.1
• A tentative spectral feature consistent with DMS is present; DMS was theoretically predicted to be an observable biomarker in Hycean worlds, but no firm detection is claimed.p.1
• The CH4 detection resolves the 'missing methane problem' for temperate exoplanets and breaks the H2O/CH4 degeneracy that rendered prior HST WFC3 (1.1–1.7 μm) results inconclusive.p.1
• NIRISS starspot occultation detected at 6.9σ (ln B = 21.6); uncorrected, it would have systematically altered the transmission spectrum. SPOTROD modelling was applied to both transit epochs.p.5
• K2-18b orbital and physical parameters: mass 8.63 ± 1.35 M⊕, radius 2.61 ± 0.09 R⊕, equilibrium temperature ~250–300 K (albedo 0–0.3), orbital period 32.94 days.p.2
• Retrieval used AURA code with free chemistry; best-fit reduced χ²ν = 1.080 on native-resolution spectra spanning 1,010 (NIRISS) and 3,401 (NIRSpec) spectral data points.p.6
• NIRSpec G395H achieves R~2700 across 2.73–5.17 μm (two detectors, gap at 3.72–3.82 μm); host star K2-18 was too bright for direct target acquisition, requiring a nearby field star (2MASS J11301306+0735116) as the acquisition target.p.3

Verbatim

• “The spectrum reveals strong detections of methane (CH 4 ) and carbon dioxide (CO 2 ) at 5 σ and 3 σ confidence, respectively, with high volume mixing ratios of ∼ 1% each in a H 2 - rich atmosphere.”

  p.1

• “The abundant CH 4 and CO 2 along with the nondetection of ammonia (NH 3 ) are consistent with chemical predictions for an ocean under a temperate H 2 -rich atmosphere on K2-18 b.”

  p.1

• “The spectrum also suggests potential signs of dimethyl sulfide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet.”

  p.1

• “The detection of CH 4 resolves the long-standing missing methane problem for temperate exoplanets and the degeneracy in the atmospheric composition of K2-18 b from previous observations.”

  p.1

• “The planet has a mass of 8 . 63 ± 1 . 35 M ⊕ and radius of 2 . 61 ± 0 . 09 R ⊕ , with an equilibrium temperature of ∼ 250-300 K for an albedo between 0-0.3”

  p.2

Most interesting

• K2-18 was too bright for NIRSpec direct target acquisition; a nearby unrelated star (2MASS J11301306+0735116) had to be used as the acquisition reference.
• The NIRISS starspot occultation was so strong (6.9σ, ln B = 21.6) that discarding the affected data was avoided entirely by fitting a spot model jointly with the transit, preserving the full 4.9-hour exposure.
• DMS on Earth is produced almost exclusively by marine phytoplankton metabolizing dimethylsulfoniopropionate; a confirmed exoplanet DMS detection would constitute the first plausible atmospheric biosignature outside the solar system.
• Prior HST WFC3 spectra of K2-18b (1.1–1.7 μm) were ambiguous between H2O and CH4 absorption for years; JWST's longer-wavelength coverage at R~2700 resolved the degeneracy in a single transit.
• Hycean worlds are predicted to have a habitable zone substantially wider than Earth-like planets, and their low gravity / low mean molecular weight atmospheres produce scale heights large enough to be detectable with modest JWST time.
• The retrieval included five candidate biosignature molecules alongside standard CNO species: DMS, CS2, CH3Cl, OCS, and N2O, a systematic biomarker survey embedded in what is nominally a composition study.